Major controversy has recently arisen regarding a possible increased incidence of sudden cardiac death in hypertensive individuals treated with diuretics. To investigate a possible etiologic mechanism for such an outcome, we compared the prevalence and complexity of exercise-induced arrhythmias in BLSA subjects on chronic diuretic monotherapy for untreated hypertension with that in normotensive control group. Although the prevalence of exercise induced arrhythmias was higher in the diuretic treated group (D) that in controls (C) 57% versus 38%, p less than .05, this difference was due entirely to the higher prevalence of simple the groups was found in the prevalence of frequent or complex ectopic beats. Furthermore, within the D group, no difference in the occurrence of ectopic beats was found between men and women, those with resting ECG abnormalities and those without or between those with serum K less than 3.7 versus those with higher serum K.